Jay Hamm is dead on. A tent is a good way to go. Remember: A little down goes a long way. And goes and goes. Any that gets loose will plague you for months. Amazing stuff.
I set up a 3-man dome and keep a canister vacuum outside with the hose, cord and extension cord running inside. Before I get out of the tent, I plug in the vac, suck up all the errant down, then exit. If you use a clean disposable vac bag, you can recover much of the loose down.
Another thing. If you stitch two parallel rows along your cutting lines, you can keep the down encapsulated. I've done this, and it works. I keep a sewing machine inside the tent. I just rip the thread on the 'waste' chambers as I need the down, and open whichever chamber I'm filling before adding down and re-stitching.